Image projection and/or viewing systems commonly called projectors or rearprojectors according to whether projection is done on the front face of the screen for projectors, or whether projection is done on the rear face for rearprojectors, operate according to the same principle. An illumination system illuminates in a uniform manner one or more imagers comprising an array of pixels, for example of the LCOS “Liquid Crystal On Silicon”, DMD “Digital Micro-mirror Device/Display”, or HTPS “High Temperature Poly Silicon” transmissive liquid crystal type, disposed for example in rows and columns on a substrate forming an active matrix, in particular made of silicon. The light emanating from the illumination system is modulated after passage through the imager or imagers in the case of transmissive imagers or after reflection on the imager or imagers in the case of reflective imagers, for example LCOS or DMD. The thus modulated light is thereafter projected onto a screen through an optical device. To generate colour images, the imager or imagers is or are illuminated with one or more beams of coloured light, generally Red, Green and Blue.
Certain image projection and/or viewing systems comprise three imagers, each of the imagers being illuminated by a coloured light beam corresponding to one of the three primary colours. These image viewing systems with three imagers are complex, it is necessary to specifically recombine the three beams of coloured light which have passed through or have been reflected on the imagers so as to recompound only a single modulated light beam ready to be projected onto a screen. In addition to the number of imagers used, these image viewing systems require the use of cumbersome and expensive optical elements. There exist image projection and/or viewing systems comprising two imagers, as for example the image projection system described in IBM patent U.S. Pat. No. 5,863,125. In this document, each imager is alternately illuminated by a coloured light beam, in such a way that the electronic addressing of the data for writing the pixels of a frame as well as the stabilization of the said pixels related to the response time of the liquid crystals, can be performed on the imager which is not illuminated. This period, of the order of a few milliseconds, during which an imager may not be illuminated, is called “dead time”. The image projection system described in the IBM document has the advantage of managing this dead time, however, it remains complex and expensive by dint of the presence of the two imagers. Moreover imagers of the LCOS “Liquid Crystal On Silicon”, or DMD “Digital Micro-mirror Device/Display” type for example exhibit less significant dead times than previously, DMD type imagers can indeed operate at high frequencies greater than four hundred Hertz for example.
For a few years now, image projection systems have been evolving towards a mono-imager device, or “mono-valve” as it is known, of transmissive or reflective type, that are less cumbersome and less expensive than image projection systems comprising several imagers.
To generate colour images, projection systems of this type sequentially display images of different colours on the screen, generally the three primary colours RGB (red, green and blue). These devices are named “Field Sequential colour displays”. These image projection systems generally comprise illumination devices for illuminating the single imager by an alternately red, green and blue light through the use, for example, of colour wheels such as described in patent application EP 0749250. These colour wheels generally comprise a red segment, a green segment, and a blue segment, R, G, B that are transmissive (coloured filters) or reflective. The video data controlling the writing of the pixels of the imager are synchronized with the colour of illumination of the imager so as to form an image ensuring the best possible restitution. However, image projection and/or viewing systems with single imager of this type have certain disadvantages relative to systems with several imagers. For example, the imager must operate at high frequencies, generally three times the frequency of the images for a colour wheel with three primary colour segments, i.e. at least 150 to 180 Hz, and generally at n times the frequency of the images when using a colour wheel with n primary colour and/or compound colour segments. Moreover, given that a single colour is displayed at one and the same time, a significant share of the light flux emanating from the light source is lost since it is filtered sequentially.
Thus, to improve the luminosity of the projected images, certain colour wheels comprise a white segment as for example the device described by Texas Instrument in patent U.S. Pat. No. 5,233,385. This device makes it possible to display brighter images, to the detriment of the quality of the colours which will be desaturated (less vivid) because of the presence of a polychromatic light covering at the minimum a part of the visible range and of white aspect.
Another major drawback related to the sequential display of images of different colours on a screen is the perception by the viewer of the decomposition or separation of the colours into these RGB primary components when for example certain objects of the image move rapidly or during an abrupt movement of the head, a blinking of eyes, or any sudden arbitrary spatial variation of the retinal image of the observed scene, whatever the observation conditions. This phenomenon called “colour break up” or the “Rainbow effect”, is manifested also for stationary images, for example when the eyes of the viewer rapidly scan the screen, when the eyes of the observer are subject to nystagmus or the phenomenon of micro-accommodation, or else for rapidly moving images with a stationary observer. One of the means for reducing this phenomenon of “colour break up” is to increase the frequency of sequential display/illumination of the imager. Certain colour wheels comprise six colour segments for example in the form of two coplanar subsets of three RGB coloured segments as described in the Texas Instrument American patent U.S. Pat. No. 5,448,314. The speed of rotation of the colour wheel being determined, this method makes it possible to double the frequency of colour sequential illumination on the imager whose addressing frequency must also be doubled. Nevertheless the “colour break up” is attenuated but still remains quite visible since the image frequency is well below the threshold defining the limit of perception of the phenomenon i.e. two thousand five hundred Hertz according to the minimum width of a colour artefact corresponding to an image pixel, and according to the visual acuity of the standard observer.